A MODEL ON THE STRUCTURE AND ORGANIZATION OF THE POLYTENE CHROMOSOME BASED ON THE STUDY ON CHIRONOMUS STRIATIPENNIS KIEFFER (DIPTERA: CHIRONOMIDAE)

Abstract

A study on the polytene chromosomes obtained from the salivary gland cells of Chironomus striatipennis Kieffer
indicated that a polytene chromosome is not a typical ribbon like body formed of a number of thread like
chromatin elements aligned side by side having condensed chromatin thread at locations forming bands, simple
linear threads of chromatin as interbands and highly extended chromatin threads forming Balbiani rings and puffs
at places as conventionally thought of, instead a chromosome appears to be a cylindrical body with thin and thick
circular bands as elevated regional differentiation of the chromatin threads having interbands in between as
depressed regions over a chromosome. The polytene chromosomes prepared from the Chironomid larvae
showed the characteristic configurations to support the organization of polytene chromosomes as conjectured.
The variable dimensions of bands appeared due to variable amount of condensed chromatin material and
intervening interbands also varied in their magnitude of extension. Besides some regions of excessive constriction
also appeared as waists along a chromosome arm and such differentiation could be possible due to presence of
some quasifluid substance making the core of the cylindrical chromosome. The multiple chromatin threads in the
polytene chromosome appeared to be arranged on the surface line of the long chromosome with coiled or kinky
orientation of the thread at identical locations in all the chromatin threads. The chromatin threads were so
arranged that the surface of the chromosome appeared smooth laterally except some swelled protrusions at
places. The swelled protrusions were the bands and the depressed regions between them were the interbands.
The inner core of the cylindrical chromosome appeared to be filled up with a material of quasi-fluid consistency.